Turbine rotors used in power generating facilities must be balanced periodically to insure smooth, efficient operation of the rotor. This balancing is generally accomplished by inserting weights around the circumference of the rotor, the mass and exact position of the weight being dictated by the imbalance. Generally three circumferential planes of the rotor are made available for balancing, one on either end of the rotor, and one in the middle, known as the center plane balance.
Because of the time and expense involved in removing the turbine casing to access the rotor, means have been devised for inserting the weights while the casing is in place. The conventional means of accessing the rotor for center plane balancing consists of a cylindrical access port leading from the outside of the casing to the rotor. During operation, this port must be closed and plugged with seals at points along its length to prevent the bypass leakage of steam which would be detrimental to turbine efficiency. This port serves no purpose other than to allow center plane balancing. To use the port, the operator must open it and remove each of the sealing plugs. Further, balancing often requires more than one weight placement, or "balance move". After each balance move, however, the port must be replugged and closed so further balancing needs can be diagnosed. Lastly, balancing the rotor can be a "critical path" operation. Therefore, a design allowing for faster access to the rotor is desirable.